Skip to main content

REPL

Running gopy with no script drops into the interactive prompt. The banner matches CPython's:

gopy 0.12.0 (3.14.0+) [go1.26 darwin/arm64]
Type "help", "copyright", "credits" or "license" for more information.
>>>

Behaviour​

Each line is parsed in mode="single". Expression statements print their value via sys.displayhook, so

>>> 2 + 2
4

is real output, not a print call.

A blank line ends a multi-line block. The classic continuation prompt is ... :

>>> def double(x):
...     return x * 2
...
>>> double(21)
42

History​

gopy uses myreadline/ for line editing. The history file path is the same one CPython uses: $XDG_STATE_HOME/python_history, falling back to ~/.python_history.

Hooks​

The REPL respects sys.displayhook, sys.excepthook, and sys.__interactivehook__. Replacing displayhook is the canonical way to customise how repr-style output is rendered.

>>> import sys
>>> def loud(value):
...     if value is None: return
...     print(f"=> {value!r}")
>>> sys.displayhook = loud
>>> 1 + 1
=> 2

Soft and hard exits​

Ctrl-D on an empty line exits cleanly. Ctrl-C raises KeyboardInterrupt at the current prompt and clears the in-progress buffer. exit() and quit() are real functions that raise SystemExit.

Line editing​

gopy's line editor is myreadline/. By default it runs the bare StdioReadline mode: prompt is drawn, input is read to newline. There is no in-place editing, no history navigation, and no tab completion in that mode.

To get a full-featured terminal experience today, pipe through rlwrap:

rlwrap gopy

A future build tag will link in peterh/liner or GNU readline.

Plugging in a custom reader​

A Go host that embeds the REPL can supply its own line reader via myreadline.SetReader. The function receives stdin / stdout plus the prompt and returns the user's line.

import "github.com/tamnd/gopy/myreadline"

myreadline.SetReader(func(stdin io.Reader, stdout io.Writer, prompt string) (string, error) {
    // Run your own line editor here.
})

This is what an embedder would use to integrate a TUI library or a network shell.

Quirks worth knowing​

  • Multi-line input ends on a blank line, not on dedent. Pasting blocks copied with the leading prompt characters (>>> , ... ) does not work yet; strip them first.
  • The displayhook only fires in mode="single". Inside a def body, expression statements at the inner level are evaluated but their values are not printed.
  • _ holds the value of the last expression printed at the prompt. Re-binding _ (a common idiom in i18n code) breaks this; the REPL never restores the binding.